Providing adequate energy to power all the various needs of society is becoming more problematic every year. Conventional sources such as coal, petroleum and natural gas are becoming more expensive and harder to find. At the same time, the byproducts of combustion produce air pollution and elevate atmospheric carbon dioxide, threatening severe consequences for global environments. Renewable sources of energy, particularly solar collectors and wind turbines, could largely replace hydrocarbons if they could be converted from intermittent production to reliable, dispatchable power supplies. This could be accomplished by directing a significant fraction of the output from solar and/or wind sources into large-scale energy storage units, which would then release that energy as needed.
The primary technology currently in use for very high capacity energy storage is pumped storage hydro, also called simply “pumped hydro”, as shown in FIG. 1. A typical installation 50 employs two large water reservoirs, with a first or low reservoir 52 at a lower elevation than a second or high reservoir 54. Hydraulic pump-turbines in a powerplant chamber 56 driven by a motor/generator pump water from the low reservoir 52 to the high reservoir 54 whenever excess energy is available. Upon demand, water is released from the high reservoir 54 and flows through the pump-turbines into the low reservoir 52 to generate electricity. Large installations can have a peak output power of more than 1000 megawatts and a storage capacity of thousands of megawatt-hours. The powerplant chamber 56 may include a separate pump and turbine both connected by drive shafts to a motor-generator. This arrangement can operate in the same fashion as the conventional design to store and release energy, but it provides another mode called the “hydraulic short circuit” that greatly increases flexibility.
Pumped hydro has been the premier bulk storage technology for decades, with over 120 gigawatts of generation capacity worldwide, but geographic, geologic and environmental constraints associated with reservoir design in addition to increased construction costs have made it much less attractive for future expansion. Thus, this technology is not a practical method to provide the wide applicability, terawatt generation capacity, low cost and environmental compatibility required to support a major conversion of the energy infrastructure from hydrocarbon to renewable sources of energy.
An alternative technology includes storing energy by using a pressurized fluid to elevate a piston in a hollow shaft. In systems with a large shaft and piston diameter, some construction techniques can produce large variations in shaft and/or piston circumference due to their limited control of tolerances. In order for a seal between the piston and the walls of the shaft to maintain the tight contact required for good seal performance, such variation in circumference would require corresponding dynamic variation in seal circumference as the piston moves up and down. This dynamic variation can be difficult to achieve with materials capable of handling high contact forces. Further, a rough contact surface for the seal can result in poor seal performance and fast seal wear. High pressure leakage past the seal through small passages in a rough contact surface can cause scouring of that surface, wearing away material and accelerating deterioration of the system.